Aspects of the behavioral ecology of the Harris's hawk (Parabuteo unicinctus) in southeastern Arizona

Abstract

I tested three models for the evolution of cooperative breeding, as applied to Harris's Hawks. The ecological constraints model argues that cooperative breeding arises in response to habitat saturation or a harsh, variable climate. I assessed group size, territory quality, and reproductive success at 45 nests during 1986 through 1990. I found that reproductive success of pairs declined when rainfall and prey declined, while reproductive success of groups remained stable, suggesting that helping is favored under harsh conditions. I found no evidence of habitat saturation. Fisherian theory suggests that parents invest more resources in individual offspring of the larger sex, i.e., the female in Harris Hawks. According to the repayment model, however, cooperative breeders preferentially invest in male offspring. Both models predict male-biased sex ratios. I sexed 87 young fledged during 1986 through 1990 and found sex ratio bias only in 1990, when all female nestlings starved. I observed feeding at seven nests with bisexual broods and found that dominant females fed female offspring more than male offspring. According to the cooperative hunting model, Harris's Hawks form groups, because groups kill larger prey, obtaining a higher per capita caloric intake. This analysis fails to consider the importance of small prey items. I compared direct observations of the prey items eaten at 41 Harris's Hawk nests with prey remains found in 18 nests and with published reports, I showed that previous reports based on prey remains were biased in favor of large prey items. Nest attendance by female birds of prey has been linked to the female's role in antipredator defense. Dominant female Harris's Hawks with helpers to assist in nest defense should therefore attend the nest less than females lacking helpers. Females rearing broods in conspicuous nests should be more attentive than females with cryptic nests. I observed nest attendance and defense at 47 nests. I found that dominant females took the leading role in antipredator defense, especially against coyotes and Turkey Vultures. Females spent more time on conspicuous nests, due to the need to shade the young from direct sunlight. Helpers had no effect on attendance.